Java 注解和反射（六）获取泛型，注解信息

反射操作泛型

**Java采用泛型擦出的机制来引入泛型，Java中的泛型仅仅是给编译器javac使用的，确保数据

　的安全性和免除强制类型转换问题，但是，一旦编译完成，所有和泛型有关的类型全部擦除

**为了通过反射操作这些类型，Java新增了ParameterizedType，GenericArrayType，TypeVariable

　和WildcardType几种类型来代表不能被归一到Class类中的类型但是又和原始类型齐名的类型

**ParmeterizedType：表示一种参数化类型，比如Collection<String>

**GenericArrayType：表示一种元素类型是参数化类型或者类型变量的数组类型

**TypeVarible：是各种类型变量的公共父接口

**WildcardType：代表一种通配符类型表达式

public class Test10 {
    public void test01(Map<String,User> map , List<User> list){
        System.out.println("test01");
    }
    public Map<String,User> test02(){
        System.out.println("test02");
        return null;
    }

    public static void main(String[] args) throws NoSuchMethodException {
        Method method = Test10.class.getMethod("test01", Map.class, List.class);
        Type[] genericParameterTypes = method.getGenericParameterTypes();
        for (Type genericParameterType : genericParameterTypes) {
            System.out.println("#"+genericParameterType);
            if (genericParameterType instanceof ParameterizedType){
                Type[] actualTypeArguments = ((ParameterizedType) genericParameterType).getActualTypeArguments();
                for (Type actualTypeArgument : actualTypeArguments) {
                    System.out.println(actualTypeArgument);
                }
            }
        }
         method = Test10.class.getMethod("test02", null);
         Type genericReturnType = method.getGenericReturnType();
        if (genericReturnType instanceof ParameterizedType){
            Type[] actualTypeArguments = ((ParameterizedType) genericReturnType).getActualTypeArguments();
            for (Type actualTypeArgument : actualTypeArguments) {
                System.out.println(actualTypeArgument);
            }
        }
    }
}

反射操作注解

getAnnotations

getAnnotation

练习ORM

了解什么是ORM？

Object relationship Mapping --> 对象关系映射

类和表结构对应

属性和字段对应

对象和记录对应

要求：利用注解和反射完成类和表结构的映射关系

//练习反射操作注解
public class Test11 {
    public static void main(String[] args) throws ClassNotFoundException, NoSuchFieldException {
        Class c1 = Class.forName("refection.Student");
        //通过反射获得注解
        Annotation[] annotations = c1.getAnnotations();
        for (Annotation annotation : annotations) {
            System.out.println(annotation);
        }
        //获得注解Value的值
        TableLuo tableLuo = (TableLuo) c1.getAnnotation(TableLuo.class);
        String value = tableLuo.value();
        System.out.println(value);

        //获得属性的注解
        Field name = c1.getDeclaredField("name");
        FieldLuo annotations1 = name.getAnnotation(FieldLuo.class);
        System.out.println(annotations1.columnName());
        System.out.println(annotations1.length());
        System.out.println(annotations1.type());
    }
}

@TableLuo("db_student")
class Student{
    @FieldLuo(columnName = "db_id",type = "int",length = 10)
    private int id;
    @FieldLuo(columnName = "db_age",type = "int",length = 10)
    private int age;
    @FieldLuo(columnName = "db_name",type = "varchar",length = 3)
    private String name;

    public Student() {
    }

    public Student(int id, int age, String name) {
        this.id = id;
        this.age = age;
        this.name = name;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return "student{" +
                "id=" + id +
                ", age=" + age +
                ", name='" + name + '\'' +
                '}';
    }
}

//类名注解
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@interface TableLuo{
    String value();
}

//属性注解
@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
@interface FieldLuo{
    String columnName();
    String type();
    int length();
}